This invention relates to a mobile communication network and, more particularly, to a synchronization system for base stations incorporated in an asynchronous transfer mode mobile communication network.
The digital communication channel is broken down into a synchronous transfer mode and an asynchronous transfer mode. The synchronous transfer mode and the asynchronous transfer mode are abbreviated as xe2x80x9cSTMxe2x80x9d and xe2x80x9cATMxe2x80x9d, respectively. In the synchronous transfer mode, time slots are periodically assigned to digital signals on different channels through the time division multiplexing, and the digital signals are transferred from a base station to a high-level station. Even if there is not any digital signal on a channel, the time slot is still periodically assigned to the channel, and the utilization factor is rather low. Therefore, the synchronous transfer mode is less appropriate for variable traffic data such as voice data and packet data. On the other hand, digital signals on different channels are formed into fixed-length cells, and the fixed-length cells are multiplexed depending upon the traffic on each channel. For this reason, the asynchronous transfer mode is more appropriate for the variable traffic data.
A mobile communication network is expected to transmit variable traffic data, and the asynchronous transfer mode is desirable for the mobile communication network. A high-level station is shared between plural base stations in the mobile communication network, and a synchronization between the base stations is required for a hand-off.
Signal transmission timings at the base stations are controlled in a time division multiple access, which is abbreviated as xe2x80x9cTDMAxe2x80x9d, system. The time division multiple access establishes a synchronization between the base stations with respect to the high-level station in the mobile communication network, and is effective against the silent time due to the hand-off.
A code division multiple access, which is abbreviated as xe2x80x9cCDMAxe2x80x9d, system is attractive to the communication service, because the user channels per unit band width are more than the user channels of the time division multiple access. The code division multiple access requires the synchronization for continuous communication at the hand-off.
Nevertheless, it is not easy to establish the synchronization between the base stations with respect to the high-level station in the asynchronous transfer mode network, because the transmission delay is always varied.
Japanese Patent Publication of Unexamined Application No. 6-30466 proposes an asynchronous transfer mode mobile communication system between plural base stations and a control station. The asynchronous transfer mode mobile communication system includes a mobile communication controlling station and wireless base stations communicable with the mobile communication controlling station through plural transmission channels. The transmission channels are respectively connected to signal processing units incorporated in the mobile communication controlling station. A channel controller is provided between modulator/demodulator units and the transmission channels. When a hand-off is required, the wireless base station needs to change a modulator/demodulator unit communicating with a mobile station to another modulator/demodulator unit, and the channel controller connects the new modulator/demodulator to the transmission channel already used. Thus, the channel controller changes the connection between the modulator/demodulator units and the transmission channels at the hand-off, and any interruption does not take place in the communication with the mobile station. However, the Japanese Patent Publication of Unexamined Application is silent to the synchronization between the base stations.
If a high-precision clock signal is shared between wireless base stations, the high-precision clock signal makes the base stations synchronous with one another with respect to the high-level station. FIG. 1 illustrates the prior art asynchronous transfer mode mobile communication system proposed as TIA/EIA/IS-95-A system. Wireless base stations 1/2 are connected through asynchronous transfer mode channels 3/4 to a high-level station 5, and a mobile station 6 is communicating with the wireless base station 1 through a radio channel 7. The global positioning system is available for the synchronization between the wireless base stations 1 and 2. The global positioning system is abbreviated as xe2x80x9cGPSxe2x80x9d, and the twenty-four space satellites broadcast high-precision clock signal indicative of the current time. GPS receivers 1a/2a are incorporated in the wireless base stations 1/2, respectively, and the high-precision clock signal establishes the synchronization between the wireless base stations 1/2.
A cesium atomic clock can provide a highly reliable clock signal, and is available for the synchronization between the base stations in the asynchronous transfer mode mobile communication system.
However, the cesium atomic clock is so expensive that the installation in each base station is not feasible. On the other hand, the GPS receiver 1a/2a is economical, and the prior art synchronization system shown in FIG. 1 is feasible in view of the cost. However, the space satellites can not eternally broadcast the high-precision clock signal. For this reason, the prior art synchronization system is less reliable.
It is therefore an important object of the present invention to provide a synchronization system for an asynchronous transfer mode mobile communication network which is economical and reliable.
To accomplish the object, the present invention proposes to periodically measure a difference in transmission time lags along asynchronous transfer mode channels.
In accordance with one aspect of the present invention, there is provided a n asynchronous transfer mode network comprising a plurality of low-level stations communicable with at least one mobile station in a boundary zone between service cells of the plurality of low-level stations, a high-level station communicable with the plurality of low-level stations through respective asynchronous transfer mode channels, and a synchronization system including a plurality of clocks respectively associated with the high-level station and the low-level stations and independently indicating current times, a message transmitting unit associated with the high-level station and concurrently transmitting a control message from the high-level station to the plurality of low-level stations through the asynchronous transfer mode channels, a plurality of message relaying units respectively associated with the plurality of low-level stations, replaying the control message from one of the plurality of low-level stations to the at least one mobile station, respectively receiving a plurality of first response messages respectively supplied from the at least one mobile station to the plurality of low-level stations and respectively transmitting a plurality of second response messages from the plurality of low-level stations through the asynchronous transfer mode channels to the high-level station, at least one first message generator associated with the at least one mobile station and transmitting the plurality of first response messages to the plurality of message relaying units after the at least one mobile station receives the control message, a plurality of second message generators respectively connected to the plurality of message relaying units and respectively producing the plurality of second response messages each representative of the current time indicated by the clock associated with one of the plurality of low-level stations and a first message transmission routine from aforesaid one of the plurality of low-level stations to the at least one mobile station and vice versa and a timing regulator connected to the message transmitting unit and determining a difference between transmitting time lags along the asynchronous transfer mode channels on the basis of the second response messages and a plurality of second message transmission routines from the high-level station to the at least one mobile station and vice versa for supplying a piece of control information representative of the difference between the transmitting time lags to the plurality of low-level stations.